Cyclic polypeptide antibiotic Virginiamycin S1 is a major component of virginiamycin complex, which is used in veterynary medicine to treat Gram-positive infections, and as a growth promoter in young swine and cattle.

Virginiamycin is originated in Streptomyces virginiae, but also other strains of streptomycettes. Virginiamycin S1 is a cyclic polypeptide antibiotic, of Streptogramin group B class. Reportedly, it targets the 60S ribosomal protein L37 affecting translation, ribosomal structure. It binds to the 23S rRNA